Agriculture in the Twentieth Century will forever be remembered because of the great advances in the genetic improvement of crops. The revolutionary appearance of hybrid corn early in the century was one such event and this was rapidly followed by enhancements in many crops using traditional breeding practices that increased tolerance and/or resistance to environmental stresses and pests. But perhaps what will be remembered most about crop improvement happened at the end of the century. Technology emerged that by-passed the time consuming traditional breeding of plants and animals, and for the first time, crops were commercially produced that had been genetically modified using techniques of molecular biology in a way not possible before. The products of this new genetic engineering technology are generally called “genetically modified organisms” or simply GMOs. In agriculture these products are often referred to as genetically modified (GM) crops. The biological processes by which GM crops are created and their incorporation in the world food supply are both complex and controversial and will not be discussed in detail in this module. However, some explanation of the processes and terms is needed to discuss the different products. The initial steps in the construction of a GM crop are to identify, isolate, and mark a gene (or genes) from a donor organism that controls the desired trait(s). For example, a desired trait is found to be controlled by a gene from a particular donor organism like a bacterium. This gene is chemically isolated and then marked by coupling it with another gene (or genes) that confers some distinctive trait such as herbicide resistance or antibody production. Marker traits are generally easy to detect and confirm the presence of the desired gene. Next comes a series of steps that involve the insertion, control, and assessment of those genes into recipient cells. The entire unique process that results in a transformed organism is collectively termed an “event.” The term “event” is sometimes equated to a “transformation.” Events are given unique identifiers such as “Bt11” or “MON810.” Even though two events can utilize the same protein, differing transformation processes result in unique products.
From 1996 to 2003, the worldwide amount of area planted to GM crops grew to at least 167 million acres (ISAAA Briefs, No. 30, 2003). During that time, herbicide tolerance in soybean, cotton, and canola was the most common genetically engineered trait planted, followed by products with insecticide resistance. The most widely used GM crop in the US is the soybean with resistance to glyphosate herbicide. In Indiana, the amount of corn planted with genetically modified resistance to insects rose from 8 percent in 2003 to 11 percent in 2004. Corn with a GM trait for herbicide resistance rose from 7 percent in 2003 to 8 percent in 2004. Soybean genetically modified for herbicide resistance fell in Indiana from 88 percent in 2003 to 87 percent in 2004 (NASS, June 2004). Soybean with a GM trait for insect resistance is not commercially available.
Currently, GM crops are identified either by their trade name, active protein, or “event.” This can be become confusing when these terms are used interchangeably. The number of commercial events in soybean is small and well understood while those in corn, particularly for insect resistance can be ambiguous. The purpose of this module is to provide a summary of most of the GM corn and soybean products currently used in the Midwest as of October 2004.
Crops that are resistant to the broad spectrum herbicide glyphosate are created by disabling a particular enzyme that is essential for plants to manufacture three required amino acids. Mammals, birds and fish do not have this enzyme and are not affected. A bacterium called Agrobacterium tumefaciens was discovered that had a variant form of that same enzyme which was unaffected by glyphosate. When the bacterial gene responsible for producing the variant enzyme was inserted into the genetic material of certain crops, those crops were likewise unaffected by exposure to glyphosate. Another broad spectrum herbicide is glufosinate. A gene from the bacterium, Streptomyces hygroscopius, was found to produce an enzyme that directly detoxifies gluphosinate. Therefore, the appearance of that “extra” gene in crops provides protection from that herbicide.
Corn and soybean genetically modified for resistance (or tolerance) to herbicides are currently limited to either glyphosate or glufosinate resistance. Corn hybrids and soybean varieties genetically engineered for resistance to glyphosate are labeled as Roundup Ready and are denoted as RR or RR2. Examples of other Roundup Ready crops typically grown outside of the eastern Midwest include cotton, rape/canola, and sugar/fodder beet. Corn hybrids resistant to glufosinate are labeled as Liberty Link products and may be denoted as LL or as GR (glufosinate resistant) Corn. Liberty herbicide tolerant soybean varieties, also denoted as LL, are not currently available.
It should be noted that other herbicide tolerant crops labeled as IMI (IR/IT), Clearfield (CL), SR/Poast Protected Corn, and STS soybeans were all developed using traditional tolerance selection breeding and are not considered GMOs.
Currently, insect resistance in GM corn and cotton are based on toxins produced by a common soil bacterium called Bacillus thuringiensis and is referred to as “Bt.” The bacterial gene that controls the formation of a toxic crystalline (Cry) protein is inserted into the genetic framework of recipient crops. Long before the introduction of transgenic technology, farmers have been using this environmentally friendly pesticide in dusts, liquid, and granule formulations. Bt products (GM and non-GM) contain an active protein that is highly specific to particular physiological conditions found in the gut of certain insects. When ingested, the active portion of the protein causes pores in the insect gut to open and allows the rapid loss of body fluids. The pest insects stop or slow down their feeding, rapidly dehydrate, and die. The physiological conditions necessary for the activation of the toxic portion of the proteins do not occur in mammals, birds, and fish and this makes Bt products safe to non-target organisms.
Although nematodes are not insects, their management is often found under insect control recommendations. The soybean cyst nematode resistance trait identified by CystX Ò was produced by traditional plant breeding practices, and soybean varieties containing this trait are not considered GM crops.
Combining or stacking traits is the “next generation” of transgenic products. Crops with multiple GM traits are now appearing in greater numbers. Within a few years, crops stacked with herbicide, insect and possibly disease resistance traits will be common.
Weed and Insect resistance to GM crops is an important issue. Growers who select these crops (particularly insect resistant crops) are under an expectation to closely follow all recommendations for resistance management. For insects, this includes planting the recommended amounts and configurations of refuges specific to certain GM crops. A list of current genetically modified, commercial GM corn traits available in the Midwest appears in Table 1.
ISAAS Briefs, No. 30. Global Status of Commercialized Transgenic Crops: 2003. Clive James.
NASS Acreage, June 30, 2004, National Agricultural Statistics Service, Agricultural Statistics Board, U.S. Department of Agriculture.
Table 1. Currently Registered GM Corn Traits for Herbicide Tolerance And Insecticide Resistance Used in the Midwest:
BCW=black cutworm, CRW=corn rootworm, ECB=European corn borer, FAW=fall armyworm, SWCB=southwest corn borer.
Acknowlegements: C. Richard Edwards and Glenn Nice, Purdue University.